The present invention relates to apparatus for feeding a mass of particulate and/or fibrous material of the type which includes a conduit communicating with screw conveyor means, reciprocating annular piston means coaxial with and partly surrounding the screw conveyor means, first drive means operatively associated with the screw conveyor means and second drive means operatively associated With the annular piston means for reciprocating the piston coaxially with the axis of the screw conveyor means.